The present invention relates to a hydraulic pressure control system for a transmission.
U.S. Pat. No. 4,735,113 discloses a continuously variable transmission including a V-belt type continuously variable transmission mechanism and a forward/reverse selector mechansim. The V-belt type continuously variable transmission mechanism includes a driver pulley and a follower pulley which are drivingly interconnected by a V-belt. The driver and follower pulleys are hydraulically operated in response to hydraulic fluid pressures supplied to a driver pulley servo cylinder chamber and a follower pulley servo cylinder chamber. Supplied always to the follower pulley servo cylinder chamber is a so-called line pressure generated by a pressure regulator valve that effects pressure regulation of pressurized hydraulic fluid discharged by a source of hydraulic fluid pressure. Hydraulic fluid pressure within the driver pulley cylinder chamber is regulated by a shift control valve. The shift control valve regulates fluid supply to the driver pully servo cylinder chamber and fluid discharge therefrom in response to an operating position of a shift actuator controlled by a control unit. The forward/reverse selector mechanism includes a forward clutch and a reverse brake which are hydraulically connected to a manual valve which is supplied with a throttle pressure generated by a throttle valve. The throttle valve is supplied with the line hydraulic fluid pressure and effects pressure reduction to deliver the throttle pressure. When the manual valve is placed at a reverse drive range, the throttle pressure is supplied to the reverse brake, while the forward clutch is drained. When the manual valve is placed at a forward drive range, the throttle pressure is supplied to the forward clutch, while the reverse brake is drained.
However, this known hydraulic control system poses a problem that if the throttle valve fails to operate, no hydraulic fluid pressure would be supplied to the forward clutch nor the reverse brake, so that the vehicle cannot continue running. More specifically, if the throttle valve becomes stuck due to dust entrained in the hydraulic fluid and fails to deliver any hydraulic fluid pressure, no hydraulic fluid pressure is supplied to the forward clutch nor the reverse brake. Thus, the vehicle cannot travel forward nor reverse.
An object of the present invention is to provide a hydraulic pressure control system for a transmission which provides a safeguard against the above-mentioned accident.